1. Field of the Invention
The present invention relates to mechanisms for heating cathodes in vacuum tubes or in plasma tubes such as used in ion lasers.
2. Description of Related Art
Plasma tubes used in ion lasers in the prior art have used directly heated AC filament cathodes. These cathodes require a current path to be coupled from the outside of the plasma tube to a filament cathode inside the tube that supports relatively large amount of alternating current. The alternating current induces heat in the cathode due to electrical resistance of the cathode to the current. By heating the cathode, electrons are injected into the plasma tube.
These directly heated filament cathodes are subject of to a variety of problems. For instance, the filaments are quite fragile. The filaments are susceptible to sagging over the lifetime of the tube and may sag into the optical path inside the tube. Also, the mounting posts for these filament cathodes are fragile.
Heavy filament umbilical leads into the plasma tube are required for direct coupled cathodes. The insulated feedthroughs for these leads leak and are complex structures.
The filament transformer for generating the current for directly heated cathodes is large and heavy. The current is not easily regulated using the high current transformers required in these systems. The AC current at frequencies commonly used, such as 60 Hertz, causes vibration of the plasma tube which can degrade the quality of laser beams generated with the plasma tube.
Alternative cathodes have been heated using direct DC current. However, the DC current filaments have current distribution problems which result in uneven heating of the cathode. The uneven heating causes hot spots and premature breakdown.
Accordingly, it is desirable to design a cathode which overcomes many of the problems of directly heated cathodes used in plasma tubes for ion lasers and other gas-discharge or vacuum tube applications.